In the production of oil and gas many tools are used downhole, and a way to control those tools from the surface or other remote location is needed. For example, in hydraulic fracturing or “fracking” it is common to frack one zone at a time and thus, fluid access to a single zone at a time is required, and typically achieved with packers, sliding sleeves, valves and the like, that allow access to the zone being fractured. As another example, production from one of several zones intersected by a well may be halted due to water invasion or steam breakthrough, while production continues from the other zones. As yet another example, one zone may be in communication with a production tubing string, while the other zones are shut in.
Due to the need to work different zones at different times, one must be able to differentially shut in one or more zones by differentially controlling various valves or sliding sleeves, and the like. Furthermore, there are many other downhole tools that are differentially controlled from the surface.
Various systems have been used to differentially control multiple downhole well tools. One type of system utilizes electrical signals to select from among multiple well tools for differential operation. Another system uses pressure pulses on hydraulic lines, with the pulses being counted by the individual tools to select particular tools for operation thereof.
However, these systems are less than ideal. Electrical systems typically have temperature limitations or are prone to conductivity and insulation problems, particularly where integrated circuits are utilized or connectors are exposed to hot, corrosive well fluids. Pressure pulse systems are typically very complex and, therefore, very expensive and susceptible to failure at multiple points.
U.S. Pat. No. 6,567,013 describes a significant advance in the differential control of downhole tools. That patent uses a series of hydraulic lines to address and actuate downhole tools. Multiple hydraulic lines are connected to a plurality of tools, and the lines used in a binary fashion to address and actuate individual tools. For example, if there are three lines A, B, C, pressure applied in a code of 001 (C line being pressurized) might mean open tool 1, and 010 (B line being pressurized) might mean close tool 1. With three lines, three tools can be opened and closed using such a binary code.
However, the coding described in U.S. Pat. No. 6,567,013 is somewhat limited, and for 6 tools, 3 lines would be required. Adding lines is certainly possible, but typically there is a limited amount of space downhole, and thus a limit to the number of lines that can be used.
As one option for allowing the control of additional tools, the U.S. Pat. No. 6,567,013 describes the use of four lines, which allowed the control of 12 actuators. However, this system was complex, each control device requiring two check valves, two relief valves and six pilot operated valves to operate. These components are less robust than needed in a downhole environment, where debris tolerance is needed. In addition, U.S. Pat. No. 6,567,013 requires 2 different pressure levels, which cannot be done in currently sub sea control systems. Thus, specialized systems are needed to provided this, contributing to costs.
Therefore, there is a need in the art to further improve the ideas presented in U.S. Pat. No. 6,567,013 and reduce the complexity so that even more tools can be controlled, but with fewer and more robust parts.